Friday, October 2, 2009

Padang Earthquake Not the Expected One

Padang, meanwhile, has indeed had its big quake. The 7.6-event on Wednesday may have killed thousands, Indonesian officials fear.

But how it happened was unexpected.

The expected earthquake would be an 8+ earthquake along the subduction zone, probably just like the big 2004 quake, or like this recent Samoa event. Instead, it was a deep interior-plate earthquake, most likely thrust, and most likely 'focused' (super quake, or 'Fist of God'). That's probably why we got so much damage from a deep earthquake.

The standard subduction earthquakes are way out in the ocean, and focus all their seismic energies down into the earth. They rarely cause shaking damage on shore. Instead, they whip up a giant wall of water!

Interior plate earthquakes are a reponse, in firmer crust, to all that shoving around, and bending, etc. That Japan nuclear plant got hit by one, the Kobe earthquake, etc. They can have huge intensities (high PGV) for their 'size', in terms of magnitude.